A backbone model for the low resolution study of the human IgGl immunoglobulin "Dob" has been proposed. Improvement of the phase determination for the M603 Fab is under way by means of density modification with solvent constraints on the 3.A map. Refinement of the atomic coordinates is contemplated. The immunoglobulin J539 Fab fragment has yielded a poor quality map at 4. A. Attempts are under way to solve the structure in a manner similar to that used for "Dob". The binding sites of M315, J539 and E109 have been built from sequence data and analogy with the M603 molecule. The models are being correlated with binding studies (M315 & J539) and with serological studies (E109). Sequence variations in immunoglobulins are being correlated with the known three-dimensional structures. These correlations could be useful in locating specificity determining residues in immunoglobulins of unknown structure. BIBOLIOGRAPHIC REFERENCES: Padlan, E.A., Davies, D.R., Pecht, I., Givol, D. and Wright, C.: Model-building Studies of Antigen-binding Sites: The Hapten-binding Site of MOPc-315. Cold Spr. Harb. Symp. on Quant. Biol., Vol. XLI, 627-637, 1977. Padlan, E.A.: Structural Basis for the Specificity of Antibody-antigen Reactions and Structural Mechanisms for the Diversification of Antigen-binding Specificities. Quarterly Review of Biophysics, 10, 35, 1977.